Enforced rules by the tag-loading code:
* No whitespace at either end (this will get trimmed)
* All-lowercase (we call .toLower on the tag)
* Is not empty after trimming (we discard the empty tag)
* Is not a duplicate (the tags form a set, not a list)

To edit tags, use a text editor, one line per tag.
Example: $TAG mytag
The game and the editor recognize the tags: They load and save tags,
but don't yet display them anywhere and don't offer to edit them.
Order of tags doesn't matter. Lix will save them alphabetically.

The level search (in the singleplayer or multiplayer browser) will find
tags, but you have to enter the exact tag completely. Half-words that
appear inside tags do not find that tag. Enter the complete tag.